[kristianp]

This is such a fun topic to think about, it's nearly science-fiction. The problem with fusion plants is that they don't exist except as small experimental reactors. I think this could be retrofitted to existing fission plants, pumping mercury 198 gas around the outside of the core (in heat-resistant pipes of course). Still an expensive proposition as you'd want to certify that it's not going to affect the safety and reaction rates of the reactor.

Another thought is that gold is a useful product, so anything that reduces the price of gold is good for the industries that use it. Are there other rare elements that are more useful, though?

[stubish]

I doubt I'll see it in my lifetime. Designing the reactors, building the prototype, building the real power plant... 30 years minimum. Then waiting another 13 years for the gold to stop being radioactive (although maybe radioactive gold sitting unmoving in a vault is worth as much as non-radioactive gold sitting unmoving in a vault if you pay your finance guys enough). And it requires high electricity prices to be profitable at all, so a bet against renewable energy generation and battery storage in that time frame (we don't have a shortage of gold). But yes, if you could create tantulum or something similar (expensive, in short supply, not artificially scarce because of hoarding)